Lysyl oxidase ("LOX") is an extracellular copper enzyme that initiates the crosslinking of collagens and elastin by catalyzing oxidative deamination of the .epsilon.-amino group in certain lysine and hydroxylysine residues of collagens and lysine residues of elastin (Smith-Mungo and Kagan (1998) Matrix Biol. 16:387-398 and Kaman in Biology of Extracellular Matrix, ed. Mecham (1986) Academic Press pp. 321-389). Lysyl oxidase has been shown to be important in a variety of cellular and physiologic processes including biogenesis of connective tissue matrices and bone resorption. A deficiency in lysyl oxidase activity is found in two X-linked, recessively inherited connective tissue disorders, the type IX variant of the Ehlers-Danlos syndrome and the Menkes syndrome, and in the X-linked, recessively inherited mottled series of allelic mutant mice (all characterized by abnormalities in copper metabolism). (Byers et al. (1980) New Engl. J. Med. 303:61-65; Royce et al. (1980) Biochemistry J. 192:579-586; Kuivaniemi et al. (1982) J. Clin. Invest. 69:730-733; Kuivaniemi et al. (1985) Amer. J. Human. Genet. 37:798-808; Peltonen et al. (1983) Biochemistry 22:6156-6163; Rowe et al. (1977) J. Biol. Chem. 252:939-942; Starcher et al. (1977) Biochem. Biophys. Res. Commun. 78:706-712; Danks in The Metabolic Basis of inherited Disease", eds. Stanbury et al. (1983), McGraw-Hill pp. 1251-1268). Increased lysyl oxidase activity has been associated with fibrotic disorders such as atherosclerosis, hypertension, and liver and pulmonary fibrosis. (Kagan, supra).
More recently there have been identified proteins having structural and/or functional similarities to lysyl oxidase. For example, a lysyl oxidase-like protein, referred to herein as "LOL", was identified from a human skin fibroblast cDNA library that contains extensive homology to several coding domains within the human lysyl oxidase mRNA which is believed to be involved in collagen maturation. (Kenyon et al. (1993) J. Biol. Chem. 268:18435-18437 and Kim et al. (1995) J. Biol. Chem. 270:7176-7182). Recent cloning and analysis of the mouse LOL gene (Kim et al. (1999) J. Cell Biochem. 72:181-188) demonstrated that steady state levels of LOL mRNA and type III procollagen mRNA increased coincidentally early in the development of liver fibrosis. In contrast, steady state levels of lysyl oxidase mRNA increased throughout the onset of hepatic fibrosis and appeared in parallel with the increased steady state levels of pro-alpha (I) collagen mRNA, suggesting that the LOL protein is involved in the development of lysine-derived cross-links in collagenous substrates. Moreover, the substrate specificity of the LOL protein may be different to that of lysyl oxidase and this difference may be collagen-type specific.
Likewise, a protein referred to herein as lysyl-oxidase related protein ("Lor") has been identified which inhibits many of the structural features of lysyl oxidase and is overexpressed in senescent fibroblasts and is believed to play a role in age-associated changes in extracellular proteins. (Saito et al. (1997) J. Biol. Chem. 272:8157-8160). Lor contains four domains referred to herein as scavenger receptor cysteine-rich domains ("SRCR domains") which are believed to be involved in binding to other cell surface proteins or extracellular molecules. The SRCR domain joins a long list of other widely distributed cysteine-containing domains found in extracellular portions of membrane proteins and in secreted proteins (Doolittle (1985) Trends Biochem. Sci. 10:233-237; Krieger in Molecular Structures of Receptors, eds. Rossow et al. (1986) Horwood, Chichester, U.K pp. 210-231). Examples include the EGF-like domain, immunoglobulin superfamily domains, the LDL receptor/complement. C9 domain, clotting factor Kringle domains, and fibronectin domains. These disulfide cross-linked domains appear to provide stable core structures that (i) are able to withstand the rigors of the extracellular environment; (ii) are well suited for a variety of biochemical tasks, often involving binding; and (iii) are readily juxtaposed to other types of domains to permit the construction of complex mosaic proteins. (Doolittle supra; Sudhof et al. (1985) Science 228:815-822). Lastly, a mouse cDNA encoding a putative protein having sequence homology to lysyl oxidase has recently been identified having the Accession No. AF053368, referred to herein as "Lor-2".
Lysyl oxidases ("LOXs") have been immunolocalized to the extracellular matrix regions of stroma surrounding early breast cancers (Decitre et al. (1998) Lab Invest. 78:143-151), with decreased expression observed in the stroma surrounding invasive breast cancers (Peyrol et al. (1997) Am. J. Pathol. 150:497-507). A progressive loss of LOX expression has also been observed during prostrate cancer progression in mice (Ren et al. (1998) Cancer Res. 58:1285-1290). These observations suggest that lysyl oxidases may function as tumor suppressors.
It has further been shown that human Lor is highly expressed in all adherent tumor cell lines examined, but not in cell lines that grow in suspension (Saito et al., supra), suggesting that LOXs can increase the adhesion properties of tumor cells. Lor expression was demonstrated to be concomitant with upregulation of type I procollagen. As adhesion properties contribute to the ability of tumor cells to colonize new sites, a tumor-promoting role for LOXs is also probable.
A greater understanding of the role which lysyl oxidase-like as well as SCRC domain containing proteins play in various disorders would lead to the determination of highly specific drug targets which would work to treat these disorders, e.g., cardiovascular disorders, a disorder arising from altered lysyl oxidase-like activity or a disorder arising from improperly regulated SRCR-domain containing protein activity giving rise to improperly regulated cellular processes.